1. Field of the Invention
The present invention relates to a linear motion rolling guide unit suitably applied for machine tools, industrial robots, precision processing equipment and testing equipment, in which sliders slide on track rails with a number of rolling elements interposed therebetween.
2. Description of the Prior Art
Linear motion rolling guide units generally have sliders mounted astride track rails with a large number of rolling elements interposed therebetween so that the sliders can slide on the track rails. The sliders can be slid forwardly and backwardly with high precision over relatively long distances.
One such conventional linear motion rolling guide unit is explained by referring to FIG. 8. The linear motion rolling guide unit, as shown in FIG. 8, includes mainly: a track rail 1 having raceway surfaces 9 formed in raceway grooves on longitudinally extending side wall surfaces 11; and a slider 20 slidably mounted astride the track rail 1. The slider 20 includes: a casing 2 which is slidable relative to the track rail 1 and has raceway surfaces 8 formed in raceway grooves at positions facing the raceway surfaces 9 and also has return passages 32; a number of balls 4 as rolling elements trapped between the facing raceway surfaces 8 and 9 to allow relative motion between the track rail and the casing; and end caps 5 attached to the longitudinal ends of the casing 2, the longitudinal direction being the direction in which the casing slides. The outer end of each end cap 5 is fitted with an end seal 19 that provides a seal between the track rail 1 and the slider 20. The balls 4 run circulating through the raceways 34 formed between the raceway surfaces 8 and 9, direction changing passages in the end caps 5 (see reference number 33 in FIG. 1), and through the return passages 32. For the purpose of sealing the gap between the casing 2 and the longitudinal side wall surfaces 11 of the track rail 1 and also the underside of the casing 2, an under seal 3 is provided to the casing 2.
The under seal 3, although it has the advantage of being simple in shape and readily manufacturable, poses some problems. That is, because the under seal 3 is thin, it is likely to be deformed by positioning errors between the casing 2 and the track rail 1 that form the raceways 8, 9 and by external forces. Thus, how the under seal 3 should be mounted to the casing 2 has long been an issue to be resolved.
One example process of mounting the under seal to the casing, as disclosed in Japanese Patent Laid-Open No. 112021/1989, is described by referring to FIG. 9 and FIG. 10. The linear motion rolling guide unit shown in FIG. 9 and FIG. 10 has basically the same construction and function as the unit shown in FIG. 8 and the parts identical with those of FIG. 8 are assigned like reference numerals.
The under seal 3 in the linear motion rolling guide unit, as shown in FIG. 9 and 10, comprises: a seal portion 7 that projects like a lip and is sealingly engageable with the side wall surface 11 of the track rail 1; a seal portion 10 that has its upper surface 21 as a sealing surface and is sealingly engageable with the undersurfaces of the casing 2 and the end caps 5; and a retaining portion 6 that keeps the rolling elements 4 in the raceways. These portions are formed into one integral structure of the same material. In this linear motion rolling guide unit, the seal portion 10 of the under seal 3 is arranged below the casing and in contact with the undersurface 12 of the casing 2, and a screw 16 is passed through a through-hole 15 formed in the under seal 3 and screwed into a threaded hole in the casing 2 to securely fix the under seal 3 to the casing 2.
With the linear motion rolling guide unit shown in FIG. 9 and 10, however, since the under seal 3 is secured at both ends to the metal casing 2 by screws 16 and hence cannot be moved relative to the casing 2, the under seal 3 formed of, for example, synthetic resin will be deformed after it is swelled by lubricating oil over a long period of use. Once the under seal 3 is deformed, the sealing contact condition between the undersurface 12 of the casing 2 and the upper surface 21 of the under seal 3 deteriorates. The seal portion 7 may also deform, degrading the contact condition between the seal portion 7 and the side wall surface 11 of the track rail 1, lowering the sealing performance of the linear motion rolling guide unit, particularly the dust-sealing effect. At the same time, the retaining portion 6 that retains the balls 4 may also deform increasing the gap between the top edge 14 of the retaining portion 6 and the ball retaining band 17, so that when the slider 20 is taken off the track rail 1, the balls 4 will no longer be able to be contained in the casing 2 because of their own weight and may fall from the casing 2.
Another method of mounting the under seal to the casing is proposed by Japanese Utility Model Laid-Open No. 1717/1986, which discloses a linear guide apparatus. This linear guide apparatus consists of: a long rail having a plurality of raceway grooves formed axially therein; a sliding member shaped like a saddle with a pair of arms and having raceway grooves facing the corresponding raceway grooves formed in the rail; a number of rolling elements that roll trapped between the raceway groove in the rail and the raceway groove in the sliding member; end caps mounted to both ends of the sliding member to form a circulating path for the rolling elements and to prevent dust from entering the rolling element portion from the ends of the sliding member; and dust-prevention members longer than the arms of the sliding member and arranged at the underside of the arms, the dust-prevention members being supported at both ends by the end caps, with one of their side edges placed in contact with the side surfaces of the rail to prevent dust from entering the rolling element portion from below. The arms of each end cap are formed with grooves on the side that comes into contact with the sliding member. The ends of the dust-prevention members are fitted into the grooves in the end caps so that the dust-prevention members are clamped and supported by the two end caps. The dust-prevention members in the linear guide correspond to the under seals in the linear motion rolling guide unit.
With the above linear guide apparatus, however, the dust-prevention members arranged at the underside of the arms are supported by the end caps. Since the dust-prevention members and the end caps are made of different materials, they generally have different thermal expansions caused by temperature changes. Therefore, if the thermal expansion of the dust-prevention member is larger than that of the end cap, a deflection occurs in the dust-prevention member, causing a gap between it and the rail or between it and the underside of the arms, which in turn deteriorates the sealing effect. As a result, dust can no longer be prevented from entering the interior of the linear guide equipment.